The actually operative reasons and the question of whether the effects can be tolerated or must be remedied depends on the conditions of use and the requirements for imaging quality of the mounted lens.
In many lens mounts known from the prior art, thermal stresses brought about by the different expansion coefficients of the material of the mount and of the optical element are to be compensated. The thermal stresses occurring between a mount ring and a lens or between an outer mount ring and an inner mount ring in which the lens is held are essentially caused by radially acting forces. To eliminate or minimize entry of forces into the lens, it is known to configure the joints, generally three joints, between the mount ring and lens, or outer mount ring and inner mount ring, such that they can compensate for the difference in expansion through deformation. For this purpose, the joints are configured to be radially soft as is known, e.g., from DE 10 2006 060 088 A1. The transmission of components of dynamic force, e.g., through a shock load, in a radial plane can also be reduced via joints which are configured to be radially soft. There are also downward limits with respect to the accompanying low mechanical flexural strength, particularly in the event of a commonly required high stiffness relative to axial forces. This is why, whatever the case, the reaction forces occurring through deformation act proportionately on the lens or possibly on the inner mount ring. It is also disadvantageous that, as the case may be, the inner mount ring and outer mount ring cannot be produced monolithically.
Also known in the art are lens mounts in which the mount ring in which the lens is held is specifically configured so that asymmetrically acting dynamic forces that have entered the mount ring via the outer circumference thereof, particularly force components acting in a radial plane or thermal forces acting radially in a rotationally asymmetrical manner, are absorbed and not transferred to the lens. To this end, in a lens mount known from EP 1 094 348 B1, elastic segments are formed at the inner circumferential surface, which elastic segments extend radially into an annular groove formed at the lens. This mount ring can be the inner mount ring of a lens mount so that the effects of the above-mentioned joints between an outer mount ring and an inner mount ring and the specific configuration of the inner mount ring can add up such that hardly any thermally induced reaction forces are transmitted to the lens.
A further reason for the occurrence of stresses in the lens mount may be the actuation of adjusting units by means of which, when a lens mount is divided into an inner mount ring and an outer mount ring, the inner mount ring in which the lens is held is adjusted within a radial plane relative to the outer mount ring. In this case, asymmetrically acting radial forces (hereinafter radial forces), or at least forces acting one-sidedly in a radial plane, are deliberately introduced into the inner mount ring to cause an at least approximately translational movement.
There are solutions in which the lens mount is radially stiff in the operative direction of the adjusting units along which a translational movement is introduced in each instance into the inner mount ring. This is the case when the adjusting units act directly on the inner mount ring or on a web which is oriented with the latter in radial direction as component part of one of the joints between outer mount ring and inner mount ring. An example of this is disclosed in DE 10 2008 063 223 B3.
Instead of this, the lens mount can also be radially soft in the operative direction of the adjusting units along which a translational movement is introduced in each instance into the inner mount ring in that the adjusting units act at correspondingly radially soft joints such that an introduced adjusting path is transmitted to the inner mount ring in a stepped down and, therefore, more sensitive manner. An example of this is given in DE 10 2007 030 579 A1.
In both cases of adjustable lens mounts, the adjusting forces, or portions of these adjusting forces, which act on the inner mount ring are introduced into the inner mount rings depending on the reaction forces in abutments formed by the further adjusting units or joints.
In all of the cases mentioned above, radial forces introduced into the inner mount rings, regardless of whether they are adjusting forces or reaction forces, can lead to deformation of the inner mount ring and, therefore, to a substantial deformation of the lens. Even a deformation in the range of a few nanometers represents a substantial deformation when it leads to intolerable impairments in imaging quality.
In terms of construction, the simplest way to at least reduce a deformation of the lens is by constructing the lens mount to be so sturdy in radial direction that no deformation could result that would be transmitted to the lens as a substantial deformation. However, this conflicts with the demand for a small installation space, an economical use of material, lightweight construction and a high natural frequency of the mounted lens.
In a monolithic optical mount according to DE 10 2008 063 223 B3, cited above, it may be assumed that the inner mount ring is constructed to be sufficiently thick radially so that the inner circumferential surface with which the mounted lens communicates is not substantially deformed. Considered strictly geometrically, the thickness could be reduced to a measure only slightly greater than the length of the radial web (referred to as foot piece in the cited publication) so that, apart from material, installation space could also be saved in radial direction.
DE 10 2006 038 634 A1 discloses a holding device for an optical element having an outer mount ring (referred to therein as base element) and a plurality of contact elements arranged so as to be distributed over the circumference of the base element and so as to be movable in radial direction. The contact elements are connected to one another in circumferential direction of the base element via at least one connection device such that the radial forces acting on the contact elements are coupled to one another. In this way, fluctuations and differences between the contact forces acting on the optical element at the contact elements are equalized. The contact elements can advantageously be connected, after installing the lens in the lens mount, via a stiffening ring which is supposed to counteract peeling stresses of the adhesive bond between the lens and the contact elements.